The present invention relates to a method of compensating for uneven output in a printing head employing an array of printing elements, and to a printing head in which this method is employed.
One type of printing head having an array of printing elements is the LED printing head used in some electrophotographic printers. The printing elements are light-emitting diodes (LEDs) that illuminate a photosensitive drum to create an electrostatic latent image, which is developed by application of toner particles, transferred to a printing medium, and fused onto the printing medium. An LED printing head has the advantage of providing high throughput, because the drum can be illuminated by all light-emitting diodes (LEDs) in the array simultaneously, forming an entire line of latent image dots at once.
An attendant disadvantage, however, is that the optical output of the array is, in general, not uniform. Without compensation, different LEDs would produce different amounts of light, leading to printing irregularities. The usual solution to this problem is to measure the optical output of each LED when the array is manufactured, and compensate for the differences by varying the driving energy supplied to each LED during printing operations.
One conventional LED printing head, for example, has a control unit storing four bits of compensation data for each LED. When driven, each LED receives an amount of driving energy determined by the compensation data. This arrangement enables the optical output of each LED to be adjusted in sixteen steps.
A problem is that some LEDs may require more compensation than four bits of compensation data can provide. This problem could be solved by adding more bits of compensation data, but that would require expensive alterations to the LED driving electronics in the printing head. In practice, an LED array having an LED that is outside the four-bit compensation range is discarded. This practice lowers the yield of the LED array manufacturing process, consequently increasing the LED array cost and the cost of the printer.
A more detailed description of this problem will be given below. Similar problems can occur in thermal printing heads and ink-Jet printing heads, in which the printing elements are heating elements or piezoelectric elements.